"I still believe Canada is a nation of peace where soldiers within its borders need not take up arms," Cpl. Branden Stevenson said in a statement Monday.
"My fellow soldiers and I remain proud and committed to watching over the Tomb of the Unknown Soldier as a strong, silent reminder of every person who made the ultimate sacrifice."
Stevenson was at the National War Memorial on the morning of Oct. 22 when Michael Zehaf Bibeau killed Cirillo and then stormed onto Parliament Hill, where he died in a gunfight in the Centre Block.
Stevenson said he is still in shock and grieving the loss of his best friend, whom he'd known since Grade 9 and whose love of the military inspired him to follow in Cirillo's footsteps.
They were always laughing and joking about something and the morning of Oct. 22 was no different, he said.
"As we stood sentry and walked the beat, one of my socks was sagging. Nathan smiled and intentionally kept our walk going so my sock would fall a little more," he said.
"He had such an amazing personality!"
Eyewitness accounts of that morning suggested Bibeau fired at Stevenson as well, but missed. Stevenson then ran to Cirillo's side to help bystanders administer first aid.
Stevenson said he's choosing to return to work to honour Cirillo and Warrant Officer Patrice Vincent, who was killed that same week in Quebec by a man with jihadist sympathies.
Funerals for both men were held last week.
Hundreds of mementoes left at the National War Memorial over the last 10 days to honour Cirillo and Vincent were removed by government workers over the weekend to prepare the site for next week's Remembrance Day ceremonies.
Stevenson said he was overwhelmed the outpouring of support.
"Nathan Cirillo was my friend, my best friend, my brother," he said. "I will miss him forever."